Deblocking filter

ABSTRACT

The invention is related to decoding of block wise coded video pictures. The determination of using de-blocking filtering between coded blocks is based on alternative characteristics compared to the characteristics used in H.264/AVC.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/250,187 filed on Oct. 13, 2005, all of which claim priority toNorwegian Patent Application No. 20051721 filed on Apr. 7, 2005 andNorwegian Patent Application No. 20044349 filed on Oct. 13, 2004. Thecontents of each of these documents are incorporated herein byreference.

FIELD OF THE INVENTION

The invention is related to decoding of block wise coded video pictures.

BACKGROUND OF THE INVENTION

Transmission of moving pictures in real-time is employed in severalapplications like e.g. video conferencing, net meetings, TV broadcastingand video telephony.

However, representing moving pictures requires bulk information asdigital video typically is described by representing each pixel in apicture with 8 bits (1 Byte) Such uncompressed video data results inlarge bit volumes, and can not be transferred over conventionalcommunication networks and transmission lines in real time due tolimited bandwidth.

Thus, enabling real time video transmission requires a large extent ofdata compression. Data compression may, however, compromise with picturequality. Therefore, great efforts have been made to develop compressiontechniques allowing real time transmission of high quality video overbandwidth limited data connections.

In video compression systems, the main goal is to represent the videoinformation with as little capacity as possible. Capacity is definedwith bits, either as a constant value or as bits/time unit. In bothcases, the main goal is to reduce the number of bits.

The most common video coding method is described in the MPEG* and H.26*standards. The video data undergo four main processes beforetransmission, namely prediction, transformation, quantization andentropy coding.

The prediction process significantly reduces the amount of bits requiredfor each picture in a video sequence to be transferred. It takesadvantage of the similarity of parts of the sequence with other parts ofthe sequence. Since the predictor part is known to both encoder anddecoder, only the difference has to be transferred. This differencetypically requires much less capacity for its representation. Theprediction is mainly based on vectors representing movements. Theprediction process is typically performed on square block sizes (e.g.16×16 pixels) . Note that in some cases, predictions of pixels based onthe adjacent pixels in the same picture rather than pixels of precedingpictures are used. This is referred to as intra prediction, as opposedto inter prediction. Consequently, when the pixels in a block are codedby means of intra prediction, the block is said to be an intra coded.

The residual represented as a block of data (e.g. 4×4 pixels) stillcontains internal correlation. A well-known method of taking advantageof this is to perform a two dimensional block transform. In H.263 an 8×8Discrete Cosine Transform (DCT) is used, whereas H.264 uses a 4×4integer type transform. This transforms 4×4 pixels into 4×4 transformcoefficients and they can usually be represented by fewer bits than thepixel representation. Transform of a 4×4 array of pixels with internalcorrelation will probability result in a 4×4 block of transformcoefficients with much fewer non-zero values than the original 4×4 pixelblock.

Direct representation of the transform coefficients is still too costlyfor many applications. A quantization process is carried out for afurther reduction of the data representation. Hence the transformcoefficients undergo quantization. A simple version of quantisation isto divide parameter values by a number—resulting in a smaller numberthat may be represented by fewer bits. It should be mentioned that thisquantization process has as a result that the reconstructed videosequence is somewhat different from the uncompressed sequence. Thisphenomenon is referred to as “lossy coding”. The outcome from thequantisation part is referred to as quantized transform coefficients.

Entropy coding implies lossless representation of different types ofparameters such as overhead data or system description, prediction data(typically motion vectors), and quantized transform coefficients fromthe quantisation process. The latter typically represent the largest bitconsumption.

The coding is performed on block wise parts of the video picture. Amacro block consists of several sub blocks for luminance (luma) as wellas for chrominance (chroma).

The present video standards H.261, H.262, H.263, H.264/AVC, MPEG1,MPEG2, MPEG4 all use blockbased coding. This means blockbased predictionfrom previously encoded and decoded pictures as well as blockbasedcoding of residual signal.

Blockbased coding/decoding has proven to be very efficient. However, oneof the drawbacks is that the reconstructed image may visible artifactscorresponding to the blocks used for prediction and residual signalcoding. This phenomenon is usually referred to as blocking or blockingartifacts.

One way of reducing the artifacts known in prior art is to add a postprocessing filter between the decoder an the display unit at thereceiver. An example of which is shown in FIG. 1. The filteringoperation takes place right before the presentation of the picture. Itis therefore a pure decoder/display issue that is unrelated to what theencoder does. In alternative solution, as shown in FIG. 2, the filter isintegrated in the coding loop. This is a more powerful approach, and isthe preferred solution in the specification ITU-T Rec. H.264|ISO/IEC14496-10 AVC, even if it implies that both encoder and decoder need todo the same operations to avoid drift in the reconstructed pictures.However, the integrated solution is a quite processor consuming i.a. asit requires a test procedure for each pixel line crossing the blockedges to be smoothed.

SUMMARY OF THE INVENTION

The present invention discloses a method in video decoding for reducingblocking artifacts between a first and a second block in a block wisecoded video picture by performing a test on pixel lines crossing aboundary and/or an extension of the boundary between the first and thesecond block and executing a de-blocking filter operation on boundaryneighboring pixels if the test indicates artifacts, wherein the methodfurther includes performing the test on a subset of the pixel linesonly, and if the test indicates artifacts, executing a de-blockingfilter operation on the boundary neighboring pixels in each of the pixellines crossing the boundary.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to make the invention more readily understandable; thediscussion that follows will refer to the accompanying drawings andtables.

FIG. 1 is a block scheme illustrating a decoder with a post-processingde-blocking filter,

FIG. 2 is a block scheme illustrating a decoder with an integratedde-blocking filter,

FIG. 3 illustrates a boundary between two blocks and lines of adjacentpixel positions crossing the boundary,

FIG. 4 shows a look-up table from the H.264/AVC specification forthreshold values

FIG. 5 shows a look-up table from the H.264/AVC specification forclipping values.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention is an improvement of prior art method for removalof blocking and quantization noise. As already mentioned, this method isdescribed in the specification ITU-T Rec. H.264|ISO/IEC 14496-10 AVC,the basics of which are described in the following.

In H.264/AVC an adaptive deblocking filter is applied in the codingloop. This means that further prediction is performed on filteredimages. The filter is designed to remove as much as possible of theblocking and quantization noise and still maintain as much of the imagecontent as possible. It is often a challenge to separate quantizationnoise and picture content. This is why the filter is highly contentadaptive and therefore complex concerning computational operations.

In FIG. 3, an edge between two picture blocks is shown. The letters cand d denotes two adjacent pixel positions on each side of the edge, andthe other letters denotes the 6 horizontal pixel positions closest tothe two first-mentioned pixels. According to H.264/AVC, pixels b c d emay be modified based on the value of each of the pixels and on thecharacterization of the edge itself. This modification is for equalizingthe above-mentioned artifacts. The modification is therefore carried outonly when artifacts are likely to occur.

Similar filter operations are performed on all the lines a, b, c, d, e,f. In the following description, the letters will be used without thenumbering 0-7.

According to H.264, a strength value (Str) is defined for the edge. Thisstrength value reflects whether artifacts are likely to occur betweentwo blocks and depends on one or more of the following situations aredetected:

-   -   a) If any of the two blocks on each side of the boundary is        intra coded, i.e. coded based on already coded blocks in present        picture.    -   b) If any of the two blocks on each side of the boundary        includes nonzero transform coefficients    -   c) If the size of motion vectors used to predict the blocks on        each side of the boundary exceeds a certain threshold.

Furthermore, to each 4×4 block a Quantization Parameter (QP) isassigned. The QP representing the edge is the maximum value of the QPsrepresenting the 2 blocks.

Several QP dependant threshold parameters are used:

α(QP)

β(QP)

γ(QP, Str)

α, β and γ are found in the look-up tables shown in FIG. 4. Table 1 isthe look-up table for determining α, β and table 2 is the look-up tablefor determining γ, which is a clipping value. Here, indexA and indexBdenotes QP, and bS=1, 2, 3 corresponds to criteria c, b, a,respectively, which are listed above. Consequently, the decidingcriteria a, b, c, also state the boundary characteristics.

Based on these values, a main test is performed determining whetherde-blocking filtering is to be carried out or not.

Filtering is performed only if:|c−d|<α(QP) and |b−c|<β(QP) and |d−e|<β(QP)

If the above statement is true, then a delta is calculated:Δ=(b−4c+4d−e)/8

This delta value is then clipped within the range (−γ,γ) As an exampleof clipping, if the quantization value is 32 and the characteristic ofthe boundary comply with criterion b, which corresponds to bS=2, thetable indicates that γ is 2. This implies that delta should be clippedwithin the interval {−2,2}. I.e. when delta is greater than 2, delta isassigned the value 2, when delta is less than −2, delta is assigned thevalue −2, and when delta lies is within {−2,2}, delta remains unchanged.

Delta is then used to calculate modified values:c′=c+Δd′=d−Δ

An additional test is performed to decide if b also is to be corrected:|a−c|<β(QP)

If this is true, then a value δ is calculated:δ=(2a−4b+c+d)/2

The value is then further clipped the value to the range (−γ′, γ′),wherein γ′ is a slight modification of γ. A modification of b is thencalculated by means of δ:b′=b+δ

The test and calculation is similar for e:|c−f|<β(QP):

If this is true another value δ is calculated:δ=(2f−4e+c+d)/2

The value is then further clipped to the range (−γ′, γ′). A modificationof e is then calculated by means of δ:e′=e+δ

The present invention is based on the standardized method describedabove, but discloses a simplification resulting in a reduction incomplexity without compromising too much with the qualitative result ofthe de-blocking operation.

Two embodiments of the invention will now described by an illustrativeexample.

The main difference compared to prior art, is that the decision test forfiltering/no-filtering preformed on one or a subset of lines crossingthe a boundary between two macro blocks applies to all the linescrossing the same boundary. If more than one “decision line” is involvedin the decision test, as will follow from the description bellow, whenconsidering the criterions a) b) c) above, not only two, but four blocksmust be taken into account. The “decision lines” will cross twodifferent boundaries, and hence involving four blocks.

As a first embodiment of the present invention, the second line in theFIG. 4 is selected as the “decision line” for the four lines 0-3crossing the boundary.

Then, filtering of all 4 edge lines are performed if:d=(|a1−2b1+c1|+|d1−2e1+f1|)<β(QP)

Otherwise no filtering is performed.

As opposed to prior art, the test for one line applies for all the linescrossing the block boundary. Thus, the value β is not necessarilydetermined in the same way as the β earlier described herein. E.g. otherlook-up tables than those depicted in FIGS. 4 and 5 may be used todetermine β.

In a second embodiment of the present invention, the third and the sixthline of in FIG. 4 in combination are used as the “decision lines” forall the eight lines 0-7 crossing the boundary.

Then, filtering of all 8 edge lines are performed if:d′=(|a2−2b2+c2|+|d2−2e2+f2|+|a5−2b5+c5|+|d5−2e5+f5|)<β′(QP)

Otherwise no filtering is performed. β′ may also be determined in stilla another way than in prior art. A typical relation to β of the firstembodiment would be β′=2β.

Further, if the above tests on the “decision line” is true, a separatecalculation is performed for each of the line for deriving delta and acorresponding corrected pixel values. This can generally be expressed asin the following, using the same notation as for the description ofprior art:Δ=(a−b−2c+2d+e−f)/4

Alternatively, for accommodating the calculation to the state of the artprocessor commonly used in codecs for videoconferencing, expressionabove could be transformed to:$\Delta = {\frac{d + \frac{a + e}{2}}{2} - \frac{c + \frac{b + f}{2}}{2}}$

The delta value is clipped the value to the value range (−γ,γ)b′=b+Δ/2c′=c+Δd′=d−Δe′=e−Δ/2

Simulations comparing the prior art H.264 filter and a filter accordingto the present invention has shown a decreased requirement forcomputational resources of about 50%, with practically the samesubjective image quality.

The description herein has been related to filtering of block artifactsin the horizontal direction, i.e. filtering pixels on both sides ofvertical block boundaries. However, the present invention is directlyapplicable to filtering of block artifacts in vertical direction. Thisimplies a 90° rotation of the pixels notation in FIG. 3.

1. A method in video decoding for reducing blocking artifacts between afirst and a second block in a block wise coded video picture byperforming a first test of adjacent blocks and a second test of pixellines crossing a boundary and/or an extension of the boundary betweenthe first and the second block and executing a de-blocking filteroperation on boundary neighboring pixels if the test indicatesartifacts, characterized in performing the second test on a subset ofthe pixel lines only, and if the test indicates artifacts, executing ade-blocking filter operation on the boundary neighboring pixels in eachof the pixel lines crossing the boundary.